CHAPTER I
THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM
Theories of the origin of vertebrates.—Importance of the central nervous system.—Evolution of tissues.—Evidence of Palæontology.—Reasons for choosing Ammocœtes rather than Amphioxus.—Importance of larval forms.—Comparison of the vertebrate and arthropod central nervous systems.—Antagonism between cephalization and alimentation.—Life-history of lamprey: not a degenerate animal.—Brain of Ammocœtes compared with brain of arthropod.—Summary.
At the present time it is no longer a debatable question whether or no Evolution has taken place. Since the time of Darwin the accumulation of facts in its support has been so overwhelming that all zoologists look upon this question as settled, and desire now to find out the manner in which such evolution has taken place. Here two problems offer themselves for investigation, which can be and are treated separately—the one dealing with the question of those laws of heredity and variation which have brought about in the past and are still causing in the present the evolution of living beings, i.e. the causes of evolution; the other concerned with the relationship of animals, or groups of animals, rather than with the causes which have brought about such relationship, i.e. the sequence of evolution.
It is the latter problem with which this book deals, and, indeed, not with the whole question at all, but only with that part of it which concerns the origin of vertebrates.
This problem of the sequence of evolution is of a twofold character: first, the finding out of the steps by which the higher forms in any one group of animals have been evolved from the lower; and secondly, the evolution of the group itself from a lower group.
In any classification of the animal kingdom, it is clear that large groups of animals exist which have so many common characteristics as to necessitate their being placed in one larger group or kingdom; thus zoologists are able to speak definitely of the Vertebrata, Arthropoda, Annelida, Echinodermata, Porifera, Cœlenterata, Mollusca, etc. In each of these groups affinities can be traced between the members, so that it is possible to speak of the progress from lower to higher members of the group, and it is conceivable, given time to work out the details, that the natural relationships between the members of the whole group will ultimately be discovered.
Thus no one can doubt that a sequence of the kind has taken place in the Vertebrata as we trace the progress from the lowest fishes to man, and already the discoveries of palæontology and anatomy give us a distinct clue to the sequence from fish to amphibian, from amphibian to reptile, from reptile to mammal on the one hand, and to bird on the other. That the different members of the vertebrate group are related to each other in orderly sequence is no longer a matter of doubt; the connected problems are matters of detail, the solution of which is certain sooner or later. The same may be said of the members of any of the other great natural groups, such as the Arthropoda, the Annelida, the Echinodermata, etc.
It is different, however, when an attempt is made to connect two of the main divisions themselves. It is true enough that there is every reason to believe that the arthropod group has been evolved from the segmented annelid, and so the whole of the segmented invertebrates may be looked on as forming one big division, the Appendiculata, all the members of which will some day be arranged in orderly sequence, but the same feeling of certainty does not exist in other cases.
In the very case of the origin of the Appendiculata we are confronted with one of the large problems of evolution—the origin of segmented from non-segmented animals—the solution of which is not yet known.
Theories of the Origin of Vertebrates.
The other large problem, perhaps the most important of all, is the question of the relationship of the great kingdom of the Vertebrata: from what invertebrate group did the vertebrate arise?
The great difficulty which presents itself in attempting a solution of this question is not so much, as used to be thought, the difficulty of deriving a group of animals possessing an internal bony and cartilaginous skeleton from a group possessing an external skeleton of a calcareous or chitinous nature, but rather the difficulty caused by the fundamental difference of arrangement of the important internal organs, especially the relative positions of the central nervous system and the digestive tube.
Fig. 1.—Arrangement of Organs in the Vertebrate (A) and Arthropod (B).
Al, gut; H, heart; C.N.S., central nervous system; V, ventral side; D, dorsal side.
Now, if we take a broad and comprehensive view of the invertebrate kingdom, without arguing out each separate case, we find that it bears strongly the stamp of a general plan of evolution derived from a cœlenterate animal, whose central nervous system formed a ring surrounding the mouth. Then when the radial symmetry was given up, and an elongated, bilateral, segmented form evolved, the central nervous system also became elongated and segmented, but, owing to its derivation from an oral ring, it still surrounded the mouth-tube, or œsophagus, and thus in its highest forms is divided into supra-œsophageal and infra-œsophageal nervous masses. These latter nervous masses are of necessity ventral to the digestive tube, because the mouth of the cœlenterate is on the ventral side. The striking characteristic, then, of the invertebrate kingdom is the situation of a large portion of the central nervous system ventrally to the alimentary canal and the piercing of the nervous system by a tube—the œsophagus—leading from the mouth to the alimentary canal. The equally striking characteristic of the vertebrate is the dorsal position of the central nervous system and the ventral position of the alimentary canal combined with the absence of any piercing of the central nervous system by the œsophagus.
So fundamentally different is the arrangement of the important organs in the two groups that it might well give rise to a feeling of despair of ever hoping to solve the problem of the Origin of Vertebrates; and, to my mind, this is the prevalent feeling among morphologists at the present time. Two attempts at solution have been made. The one is associated with the name of Geoffrey St. Hilaire, and is based on the supposition that the vertebrate has arisen from the invertebrate by turning over on its back, swimming in this position, and so gradually converting an originally dorsal surface into a ventral one, and vice versâ; at the same time, a new mouth is supposed to have been formed on the new ventral side, which opened directly into the alimentary canal, while the old mouth, which had now become dorsal, was obliterated.
The other attempt at solution is of much more recent date, and is especially associated with the name of Bateson. It supposes that bilaterally symmetrical, elongated, segmented animals were formed from the very first in two distinct ways. In the one case the digestive tube pierced the central nervous system, and was situated dorsally to its main mass. In the other case the segmented central nervous system was situated from the first dorsally to the alimentary canal, and was not pierced by it. In the first case the highest result of evolution led to the Arthropoda; in the second case to the Vertebrata.
Neither of these views is based on evidence so strong as to cause universal acceptance. The great difficulty in the way of accepting the second alternative is the complete absence of any evidence, either among animals living on the earth at the present day or among those known to have existed in the past, of any such chain of intermediate animal forms as must, on this hypothesis, have existed in order to link together the lower forms of life with the vertebrates.
Fig. 2.—Larval Balanoglossus (from the Royal Natural History).
It has been supposed that the Tunicata and the Enteropneusta (Balanoglossus) (Fig. 2) are members of this missing chain, and that in Amphioxus the vertebrate approaches in organization to these low invertebrate forms. The tunicates, indeed, are looked upon as degenerate members of an early vertebrate stock, which may give help in picturing the nature of the vertebrate ancestor but are not themselves in the direct line of descent. Balanoglossus is supposed to have arisen from the Echinodermata, or at all events to have affinities with them, so that to fill up the enormous gap between the Echinodermata and the Vertebrata on this theory there is absolutely nothing living on the earth except Balanoglossus, Rhabdopleura, and Cephalodiscus. The characteristics of the vertebrate upon which this second theory is based are the notochord, the respiratory character of the anterior part of the alimentary canal, and the tubular nature of the central nervous system; it is claimed that in Balanoglossus the beginnings of a notochord and a tubular central nervous system are to be found, while the respiratory portion of the gut is closely comparable to that of Amphioxus.
The strength of the first theory is essentially based on the comparison of the vertebrate central nervous system with that of the segmented invertebrate, annelid or arthropod. In the latter the central nervous system is composed of—
1. The supra-œsophageal ganglia, which give origin to the nerves of the eyes and antennules, i.e. to the optic and olfactory nerves, for the first pair of antennæ are olfactory in function. These are connected with the infra-œsophageal ganglia by the œsophageal commissures which encircle the œsophagus.
2. The infra-œsophageal ganglia and the two chains of ventral ganglia, which are segmentally-arranged sets of ganglia. Of these, each pair gives rise to the nerves of its own segment, and these nerves are not nerves of special sense as are the supra-œsophageal nerves, but motor and sensory to the segment; nerves by the agency of which food is taken in and masticated, respiration is effected, and the animal moves from place to place.
In the vertebrate the central nervous system consists of—
1. The brain proper, from which arise only the olfactory and optic nerves.
Fig. 3.—Vertebrate Central Nervous System compared with the Central Nervous System and Alimentary Canal of the Arthropod.
A. Vertebrate central nervous system. S. Inf. Br., supra-infundibular brain; I. Inf. Br., infra-infundibular brain and cranial segmental nerves; C.Q., corpora quadrigemina; Cb., cerebellum; C.C., crura cerebri; C.S., corpus striatum; Pn., pineal gland.
B. Invertebrate central nervous system. S. Œs. G., supra-œsophageal ganglia; I. Œs. G., infra-œsophageal ganglia; Œs. Com., œsophageal commissures.2. The region of the mid-brain, medulla oblongata, and spinal cord; from these arises a series of nerves segmentally arranged, which, as in the invertebrate, gives origin to the nerves governing mastication, respiration, and locomotion.
Further, the vertebrate central nervous system possesses the peculiarity, found nowhere else, of being tubular, and the tube is of a striking character. In the spinal region it is a small, simple canal of uniform calibre, which at the front end dilates to form the ventricles of the region of the brain. From that part of this dilated portion, known as the third ventricle, a narrow tube passes to the ventral surface of the brain. This tube is called the infundibulum, and, extraordinary to relate, lies just anteriorly to the exits of the third cranial or oculomotor nerves; in other words, it marks the termination of the series of spinal and cranial segmental nerves. Further, on each side of this infundibular tube are lying the two thick masses of the crura cerebri, the strands of fibres which connect the higher brain-region proper with the lower region of the medulla oblongata and spinal cord. Not only, then, are the nerve-masses in the two systems exactly comparable, but in the very place where the œsophageal tube is found in the invertebrate, the infundibular tube exists in the vertebrate, so that if the words infundibular and œsophageal are taken to be interchangable, then in every respect the two central nervous systems are comparable. The brain proper of the vertebrate, with its olfactory and optic nerves, becomes the direct descendant of the supra-œsophageal ganglia; the crura cerebri become the œsophageal commissures, and the cranial and spinal segmental nerves are respectively the nerves belonging to the infra-œsophageal and ventral chain of ganglia.
This overwhelmingly strong evidence has always pointed directly to the origin of the vertebrate from some form among the segmented group of invertebrates, annelid or arthropod, in which the original œsophagus had become converted into the infundibulum, and a new mouth formed. So far, the position of this school of anatomists was extremely sound, for it is impossible to dispute the facts on which it is based. Still, however, the fact remained that the gut of the vertebrate lies ventrally to the nervous system, while that of the invertebrate lies dorsally; consequently, since the infundibulum was in the position of the invertebrate œsophagus, it must originally have entered into the gut, and since the vertebrate gut was lying ventrally to it, it could only have opened into that gut in the invertebrate stage by the shifting of dorsal and ventral surfaces. From this argument it followed that the remains of the original mouth into which the infundibulum, i.e. œsophagus, opened were to be sought for on the dorsal side of the vertebrate brain. Here in all vertebrates there are two spots where the roof of the brain is very thin, the one in the region of the pineal body, and the other constituting the roof of the fourth ventricle. Both of these places have had their advocates as the position of the old mouth, the former being upheld by Owen, the latter by Dohrn.
The discovery that the pineal body was originally an eye, or, rather, a pair of eyes, has perhaps more than anything else proved the impossibility of accepting this reversal of surfaces as an explanation of the genesis of the vertebrate from the annelid group. For whereas a pair of eyes close to the mid-dorsal line is not only likely enough, but is actually found to exist among large numbers of arthropods, both living and extinct, a pair of eyes situated close to the mid-ventral line near the mouth is not only unheard of in nature, but so improbable as to render impossible the theory which necessitates such a position.
Yet this very discovery gives the strongest possible additional support to the close identity in the plan of the central nervous system of vertebrate and appendiculate.
A truly paradoxical situation! The very discovery which may almost be said to prove the truth of the hypothesis, is the very one which has done most to discredit it, because in the minds of its authors the only possible solution of the transition from the one group to the other was by means of the reversal of surfaces.
Still, as already said, even if the theory advanced to explain the facts be discredited, the facts remain the same; and still to this day an explanation is required as to why such extraordinary resemblances should exist between the two nervous systems, unless there is a genetic connection between the two groups of animals. An explanation may still be found, and must be diligently sought for, which shall take into account the strong evidence of this relationship between the two groups, and yet not necessitate any reversal of surfaces. It is the object of this book to consider the possibility of such an explanation.
What are the lines of investigation most likely to meet with success? Is it possible to lay down any laws of evolution? It is instructive to consider the nature of the investigations which have led to the two theories just mentioned, for the fundamental starting-point is remarkably different in the two cases. The one theory is based upon the study of the vertebrate itself, and especially of its central nervous system, and its supporters and upholders have been and are essentially anatomists, whose chief study is that of vertebrate and human anatomy. The other theory is based upon the study of the invertebrate, and consists especially of an attempt to find in the invertebrate some structure resembling a notochord, such organ being considered by them as the great characteristic of the vertebrate; indeed, so much is this the case, that a large number of zoologists speak now of Chordata rather than of Vertebrata, and in order to emphasize their position follow Bateson, and speak of the Tunicata as Uro-chordata, of Amphioxus as Cephalo-chordata, of the Enteropneusta as Hemi-chordata, and even of Actinotrocha (to use Masterman's term), as Diplo-chordata.
The upholders of this theory lay no stress on the nature of the central nervous system in vertebrates, they are essentially zoologists who have made a special study of the invertebrate rather than of the vertebrate.
Of these two methods of investigating the problem, it must be conceded that the former is more likely to give reliable results. By putting the vertebrate to the question in every possible way, by studying its anatomy and physiology, both gross and minute, by inquiring into its past history, we can reasonably hope to get a clue to its origin, but by no amount of investigation can we tell with any certainty what will be its future fate; we can only guess and prophesy in an uncertain and hesitating manner. So it must be with any theory of the origin of vertebrates, based on the study of one or other invertebrate group. Such theory must partake rather of the nature of prophecy than of deduction, and can only be placed on a firm basis when it so happens that the investigation of the vertebrate points irresistibly to its origin from the same group; in fact, "never prophesy unless you know."
The first principle, then, I would lay down is this: In order to find out the origin of vertebrates, inquire, in the first place, of the vertebrate itself.
Importance of the Central Nervous System.
Does the history of evolution pick out any particular organ or group of organs as more necessary than another for upward progress? If so, it is upon that organ or group of organs that special stress must be laid.
Since Darwin wrote the "Origin of Species," and laid down that the law of the 'survival of the fittest' is the factor upon which evolution depends, it has gradually dawned upon the scientific mind that 'the fittest' may be produced in two diametrically opposite ways: either by progress upwards to a superior form, or by degeneration to a lower type of animal. The principle of degeneration as a factor in the formation of groups of animals, which are thereby enabled to survive, is nowadays universally admitted. The most striking example is to be found in the widely distributed group of Tunicata, which live, in numbers of instances, a sedentary life upon the rocks, have the appearance of very low forms of animal life, propagate by budding, have lost all the characteristics of higher forms, and yet are considered to be derived from an original vertebrate stock. Such degenerate forms remain degenerate, and are never known to regenerate and again to reach the higher stage of evolution from which they arose. Such forms are of considerable interest, but cannot help, except negatively, to decide what factor is especially important for upward progress.
At the head of the animal race at the present day stands man, and in mankind itself some races are recognized as higher than others. Such recognition is given essentially on account of their greater brain-power, and without doubt the great characteristic which puts man at the head is the development of his central nervous system, especially of the region of the brain. Not only is this point most manifest in distinguishing man from the lower animals, but it applies to the latter as well. By the amount of convolution of the brain, the amount of grey matter in the cerebral hemispheres, the enlargement and increasing complexity of the higher parts of the central nervous system, the anthropoid apes are differentiated from the lower forms, and the higher mammals from the lower. In the recent work of Elliot Smith, and of Edinger, most conclusive proof is given that the upward progress in the vertebrate phylum is correlated with the increase of brain-power, and the latter writer shows how steady and remarkable is the increase in substance and in complexity of the brain-region as we pass from the fishes, through the amphibians and reptiles, to the birds and mammals.
The study of the forms which lived on the earth in past ages confirms and emphasizes this conclusion, for it is most striking to see how small is the cranium among the gigantic Dinosaurs; how in the great reptilian age the denizens of the earth were far inferior in brain-power to the lords of creation in after-times.
What applies to the vertebrate phylum applies also to the invertebrate groups. Here also an upward progress is recognized as we pass from the sponges to the arthropods—a progress which is manifested, first by the concentration of nervous material to form a central nervous system, and then by the increase in substance and complexity of that nervous system to form a higher and a higher type, until the culmination is reached in the nervous system of the scorpions and spiders. No upward progress is possible with degeneration of the central nervous system, and in all those cases where a group owes its existence to degeneration, the central nervous system takes part in the degeneration.
This law of the paramount importance of the growth of the central nervous system for all upward progress in the evolution of animals receives confirmation from the study of the development of individuals, especially in those cases where a large portion of the life of the animal is spent in a larval condition, and then, by a process of transformation, the larva changes into the adult form. Such cases are well known among Arthropoda, the familiar instance being the change from the larval caterpillar to the adult imago. Among Vertebrata, the change from the tadpole to the frog, from the larval form of the lamprey (Ammocœtes) to the adult form (Petromyzon), are well-known instances. In all such cases the larva shows signs of having attained a certain stage in evolution, and then a remarkable transformation takes place, with the result that an adult animal emerges, whose organization reaches a higher stage of evolution than that of the larva.
This transformation process is characterized by a very great destruction of the larval tissues and a subsequent formation of new adult tissues. Most extensive is the destruction in the caterpillar and in the larval lamprey. But one organ never shares in this process of histolysis, and that is the central nervous system; amidst the ruins of the larva it remains, leading and directing the process of re-formation. In the Arthropoda, the larval alimentary canal may be entirely destroyed and eaten up by phagocytes, but the central nervous system not only remains intact but increases in size, and by the concentration and cephalization of its infra-œsophageal ganglia forms in the adult a central nervous system of a higher type than that of the larva.
So, too, in the transformation of the lamprey, there is not the slightest trace of any destruction in the central nervous system, but simply a development and increase in nervous material, which results in the formation of a brain region more like that of the higher vertebrates than exists in Ammocœtes.
In these cases the development is upward—the adult form is of a higher type than that of the larva. It is, however, possible for the reverse to occur, so that the individual development leads to degeneration, not to a higher type. Instances are seen in the Tunicata, and in various parasitic arthropod forms, such as Lernæa, etc. In these cases, the transformation from the larval to the adult form leads to degradation, and in this degradation the central nervous system is always involved.
It is perhaps a truism to state that upward progress is necessarily accompanied by increased development of the central nervous system; but it is necessary to lay special stress upon the importance of the central nervous system in all problems of evolution, because there is, in my opinion, a tendency at the present time to ignore this factor to too great an extent.
The law of progress is this—The race is not to the swift, nor to the strong, but to the wise.
This law carries with it the necessary corollary that the immediate ancestor of the vertebrate must have had a central nervous system nearly approaching that of the lowest undegenerated vertebrate. Among all the animals living on the earth at the present time, the highest invertebrate group, the Arthropoda, possesses a central nervous system most closely resembling that of the vertebrate.
The law, then, of the paramount importance of a steady development of the central nervous system for the upward progress of the animal kingdom, points directly to the arthropod as the most probable ancestor of the vertebrate.
Evolution of Tissues.
In the whole scheme of evolution we can recognize, not only an upward progress in the organization of the animal as a whole, but also a distinct advance in the structure of the tissues composing an individual, which accompanies that upward progress. Thus it is possible to speak of an evolution of the supporting tissues from the simplest form of connective tissue up to cartilage and thence to bone; of the contractile tissues, from the simplest contractile protoplasm to unstriped muscle, and thence to the highest forms of striated muscle; of the nervous connecting strands, from undifferentiated to fine strands, then to thicker, more separated ones, resembling non-medullated fibres, and finally to well-differentiated separate fibres, each enclosed in a medullated sheath.
In the connective tissue group, bone is confined to the vertebrates, cartilage is found among invertebrates, and the closest resemblance to vertebrate embryonic or parenchymatous cartilage is found in the cartilage of Limulus. Also, as Gegenbaur has pointed out, Limulus, more than any other invertebrate, possesses a fibrous connective tissue resembling that of vertebrates.
In the muscular group, Biedermann, who has made a special study of the physiology of striated muscle, says that among invertebrates the striated muscle of the arthropod group resembles most closely that of the vertebrate.
In the nervous group the resemblance between the nerve-fibres of Limulus and Ammocœtes, both of which are devoid of any marked medullary sheath, is very apparent, and Retzius points out that the only evidence of medullation, so characteristic of the vertebrates, is found in a species of prawn (Palæmon). In all these cases the nearest resemblance to the vertebrate tissues is to be found in the arthropod.
The Evidence of Palæontology.
Perhaps the most important of all the clues likely to help in the solution of the origin of vertebrates is that afforded by Geology, for although the geological record is admittedly so imperfect that we can never hope by its means alone to link together the animals at present in existence, yet it does undoubtedly point to a sequence in the evolution of animal forms, and gives valuable information as to the nature of such sequence. In different groups of animals there are times when the group can be spoken of as having attained its most flourishing period. During these geological epochs the distribution of the group was universal, the numbers were very great, the number of species was at the maximum, and some of them had attained a maximal size. Such races were at that time dominant, and the struggle for existence was essentially among members of the same group. At the present time the dominant race is man, and the struggle for existence is essentially between the members of that race, and not between them and any inferior race.
The effect of such conditions is, as Darwin has pointed out, to cause great variation in that group; in consequence of that variation and that dominance the evolution of the next higher group is brought about from some member of the dominant group. Thus the present age is the outcome of the Tertiary period, a time when giant mammals roamed the earth and left as their successors the mammals of the present day; a time of dominance of quadruped mammals; a time of which the period of maximum development is long past, and we now see how the dominance of the biped mammal, man, is accompanied by the rapid diminution and approaching extermination of the larger mammals. No question can possibly arise as to the immediate ancestor of the biped mammal; he undoubtedly arose from one of the dominant quadrupedal mammals.
Passing along to the next evidence of the rocks, we find an age of reptiles in the Mesozoic period. Here, again, the number and variety is most striking; here, again, the size is enormous in comparison with that of the present-day members of the group. This was the dominant race at the time when the birds and mammals first appeared on the earth, and anatomists recognize in these extinct reptilian forms two types; the one bird-like, the other more mammalian in character. From some members of the former group birds are supposed to have been evolved, and mammals from members of the other group. There is no question of their origin directly from lower fish-like forms; the time of their appearance on the earth, their structure, all point irresistibly to the same conclusion as we have arrived at from the consideration of the origin of the biped from the quadruped mammal, viz. that birds and mammals arose, in consequence of the struggle for existence, from some members of the reptilian race which at that time was the dominant one on earth.
Passing down the geological record, we find that when the reptiles first appear in the Carboniferous age there is abundant evidence of the existence of numbers of amphibian forms. At this time the giant Labyrinthodonts flourished. Here among the swamps and marshes of the coal-period the prevalent vertebrate was amphibian in structure. Their variety and number were very great, and at that period they attained their greatest size. Here, again, from the geological record we draw the same conclusion as before, that the reptiles arose from the race which was then predominant on the earth—the Amphibia.
Fig. 4.—Plan of Geological Strata. (From Lankester.)
Again, another point of great interest is seen here, and that is that these Labyrinthodonts, as Huxley has pointed out, possess characters which bring them more closely than the amphibians of the present day into connection with the fishes; and further, the fish-like characters they possessed are those of the Ganoids, the Marsipobranchs, the Dipnoans, and the Elasmobranchs, rather than of the Teleosteans.
Now, it is a striking fact that the ancient fishes at the time when the amphibians appeared had not reached the teleostean stage. The ganoids and elasmobranchs swarmed in the waters of the Devonian and Carboniferous times. Dipnoans and marsipobranchs were there, too, in all probability, but teleosteans do not appear until the Mesozoic period. The very kinds of fish, then, which swarmed in the seas at that time, and were the predominant race before the Carboniferous epoch, are those to which the amphibians at their first appearance show the closest affinity. Here, again, the same law appears; from the predominant race at the time, the next higher race arose, and arose by a most striking modification, which was the consequence of altering the medium in which it lived. By coming out of the water and living on the land, or, rather, being able to live partly on land and partly in the water, by the acquisition of air-breathing respiratory organs or lungs in addition to, and instead of, water-breathing organs or gills, the amphibian not only arose from the fish, but made an entirely new departure in the sequence of progressive forms.
This was a most momentous step in the history of evolution—one fraught with mighty consequences and full of most important suggestions.
From this time onwards the struggle for existence by which upward progress ensued took place on the land, not in the sea, and, as has been pointed out, led to the evolution of reptiles from amphibians, birds and quadrupedal mammals from reptiles, and man from quadrupeds. In the sea the fishes were left to multiply and struggle among themselves, their only opponents being the giant cephalopods, which themselves had been evolved from a continual succession of the Mollusca. For this reason the struggle for existence between the fishes and the higher race evolved from them did not take place until some members of that higher race took again to the water, and so competed with the fish-tribe in their own element.
Another most important conclusion to be derived from the uprising of the Amphibia is that at that time there was no race of animals living on the land which had a chance against them. No race of land-living animals had been evolved whose organization enabled them to compete with and overcome these intruders from the sea in the struggle for existence. For this reason that the whole land was their own, and no serious competition could arise from their congeners, the fish, they took possession of it, and increased mightily in size; losing more and more the habit of going into the water, becoming more and more truly terrestrial animals. Henceforth, then, in trying to find out the sequence of evolution, we must leave the land and examine the nature of the animals living in the sea; the air-breathing animals which lived on the land in the Upper Silurian and Devonian times cannot have reached a stage of organization comparable with that of the fishes, seeing how easily the amphibians became dominant.
We arrive, then, at the conclusion that the ancestors of the fishes must have lived in the sea, and applying still the same principles that have held good up to this time, the ancestors of the fishes must have arisen from some member of the race predominant at the time when they first appeared, and also the earliest fishes must have much more closely resembled the ancestral form than those found in later times or at the present day.
What, then, is the record of the rocks at the time of the first appearance of fish-like forms? What kind of fishes were they, and what was the predominant race at the time?
We have now reached the Upper Silurian and Lower Devonian times, and most instructive and suggestive is the revelation of the rocks. Here, when the first vertebrates appeared, the sea was peopled with corals, brachiopods, early forms of cephalopods, and other invertebrates; but, above all, with the great tribe of trilobites (Fig. 6) and their successors. From the trilobites arose, as evidenced by their larval form, the king-crab group, called the Xiphosura (Fig. 5). Closely connected with them, and forming intermediate stages between trilobites and king-crabs, numerous forms have been discovered, known as Belinurus, Prestwichia, Hemiaspis, Bunodes, etc. (Fig. 5 and Fig. 12). From them also arose the most striking group of animals which existed at this period—the giant sea-scorpions, or Gigantostraca. This group was closely associated with the king-crabs, and the two groups together are classified under the title Merostomata.
The appearance of these sea-scorpions is given in Figs. 7 and 8, representing Stylonurus, Slimonia, Pterygotus, Eurypterus. They must have been in those days the tyrants of the deep, for specimens of Pterygotus have been found over six feet in length.
At this time, then, by every criterion hitherto used, by the multitude of species, by the size of individual species, which at this period reached the maximum, by their subsequent decay and final extinction, we must conclude that these forms were in their zenith, that the predominant race at this time was to be found in this group of arthropods. Just previously, the sea swarmed with trilobites, and right into the period when the Gigantostraca flourished, the trilobites are still found of countless forms, of great difference in size. The whole period may be spoken of as the great trilobite age, just as the Tertiary times form the mammalian age, the Mesozoic times the reptilian age, etc. From the trilobites the Gigantostraca and Xiphosura arose, as evidenced by the embryology of Limulus, and, therefore, in the term trilobite age would be included the whole of those peculiar forms which are classified by the names Trilobita, Gigantostraca, Xiphosura, etc. Of all these the only member alive at the present time is Limulus, or the King-Crab.
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Fig. 6.—A Trilobite (Dalmanites) (after Pictet). Dorsal view.
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Fig. 7.—Eurypterus remipes (after Nieskowski). Dorsal view.
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As, however, the term 'trilobite' does not include the members of the king-crab or sea-scorpion groups, it is advisable to use some other term to represent the whole group. They cannot be called crustaceans or arachnids, for in all probability they gave origin to both; the nearest approach to the Trilobite stage of development at the present time is to be found perhaps in Branchipus (Fig. 10) and Apus (Fig. 9), just as the nearest approach to the Eurypterid form is Limulus. Crustaceans such as crabs and lobsters are of much later origin, and do not occur in any quantity until the late Mesozoic period. The earliest found, a kind of prawn, occurs in the Carboniferous age.
Korschelt and Heider have accordingly suggested the name Palæostraca for this whole group, and Protostraca for the still earlier arthropod-like animals which gave origin to the trilobites themselves. This name I shall adopt, and speak, therefore, of the Palæostraca as the dominant race at the time when vertebrates first appeared.
If, then, there is no break in the law of evolution here, the race which was predominant at the time when the vertebrate first appeared must have been that from which the first fishes arose, and these fishes must have resembled, not the crustacean proper, or the arachnid proper, but a member of the palæostracan group. Moreover, just as the Labyrinthodonts show special affinities to the fishes which were then living, so we should expect that the forms of the earliest fish would resemble the arthropodan type dominant at the time more closely than the fish of a later era.
At first sight it seems too great an absurdity even to imagine the possibility of any genetic connection between a fish and an arthropod, for to the mind's eye there arises immediately the picture of a salmon or a shark and a lobster or a spider. So different in appearance are the two groups of animals, so different their methods of locomotion, that it is apparently only an inmate of a lunatic asylum who could possibly suggest such a connection. Much more likely is it that a fish-like form should have been developed out of a smooth, wriggling, worm-like animal, and it is therefore to the annelids that the upholders of the theory of the reversal of surfaces look for the ancestor of the vertebrate.
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Fig. 9.—Apus (from the Royal Natural History). Dorsal view. |
Fig. 10.—Branchipus stagnalis. (From Claus.) |
We must endeavour to dismiss from our imagination such forms as the salmon and shark as representatives of the fish-tribe, and the lobster and spider of the arthropods, and try to picture the kind of animals living in the seas in the early Devonian and Upper Silurian times, and then we find, to our surprise, that instead of the contrast between fishes and arthropods being so striking as to make any comparison between the two seem an absurdity, the difficulty in the last century, and even now, is to decide in many cases whether a fossil is an arthropod or a fish.
I have shown what kind of animal the palæostracan was like. What information is there of the nature of the earliest vertebrate?
The most ancient fishes hitherto discovered have been classified by Lankester and Smith Woodward into the three orders, Heterostraci, Osteostraci, and Antiarcha. Of these the Heterostraci contain the genera Pteraspis and Cyathaspis, and are the very earliest vertebrates yet discovered, being found in the Lower Silurian. The Osteostraci are divided into the Cephalaspidæ, Tremataspidæ, etc., and are found in the Upper Silurian and Devonian beds. The Antiarcha, comprising Pterichthys and Bothriolepis, belong to the Devonian and are not found in Silurian deposits. This, then, is the order of their appearance—Pteraspis, Cephalaspis, and Pterichthys.
In none of these families is there any resemblance to an ordinary fish. In no case is there any sign of vertebræ or of jaws. They, like the lampreys, were all agnathostomatous. Strange indeed is their appearance, and it is no wonder that there should have been a difficulty in deciding whether they were fish or arthropod. Their great characteristic is their buckler-plated cephalic shield, especially conspicuous on the dorsal side of the head. Figs. 11, 14, 15, 16, give the dorsal shields of Pteraspis, Auchenaspis, Pterichthys, and Bothriolepis.
In 1904, Drevermann discovered a mass of Pteraspis Dunensis embedded in a single stone, showing the same kind of head-shield as P. rostrata, but the rostrum was longer and the spine at the extremity of the head-shield much longer and more conspicuous. The whole shape of the animal as seen in this photograph recalls the shape of a Hemiaspid rather than of a fish. It is, then, natural enough for the earlier observers to have looked upon such a fossil as related to an arthropod rather than a fish.
Fig. 11.—Pteraspis dunensis (from Drevermann). Dorsal view of body and spine on the right side. Head-end, showing long rostrum on the left side.
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Fig. 12.—Bunodes lunula. (From Schmidt.) |
Fig. 13.—Auchenaspis (Thyestes) verrucosus, natural size. (From Woodward.) |
In Figs. 12 and 13 I have placed side by side two Silurian fossils which are found in the same geological horizon. They are both life size and possess a general similarity of appearance, yet the one is a Cephalaspidian fish known by the name of Auchenaspis or Thyestes verrucosa, the other a Palæostracan called Bunodes lunula.
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Fig. 14.—Dorsal Head-shield of Thyestes (Auchenaspis) verrucosus. (From Rohon.)
Fro., narial opening; l.e., lateral eyes; gl., glabellum or plate over brain; Occ., occipital region.
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Fig. 15.—Pterichthys. |
In a later chapter I propose to discuss the peculiarities and the nature of the head-shields of these earliest fishes, in connection with the question of the affinities of the animals which bore them. At this point of my argument I want simply to draw attention to the undoubted fact of the striking similarity in appearance between the earliest fishes and members of the Palæostraca, the dominant race of arthropods which swarmed in the sea at the time: a similarity which could never have been suspected by any amount of investigation among living forms, but is immediately revealed when the ages themselves are questioned.
Fig. 16.—Bothriolepis. (After Patten.)
An., position of anus.
I have not reproduced any of the attempted restorations of these old forms, as usually given in the text-books, because all such restorations possess a large element of fancy, due to the personal bias of the observer. I have put in Rohon's idea of the general shape of Tremataspis (Fig. 17) in order to draw attention to the lamprey-like appearance of the fish according to his researches (cf. Fig. 18).
Fig. 17.—Restoration of Tremataspis. (After Rohon, slightly modified.)
Fig. 18.—Ammocœtes.
The argument, then, from geology, like that from comparative anatomy and from the consideration of the importance of the central nervous system in the upward development of the animal race, not only points directly to the arthropod group as the ancestor of the vertebrate, but also to a distinct ancient type of arthropod, the Palæostracan, the only living example of which is the King-Crab or Limulus; while the nearest approach to the trilobite group among living arthropods are Branchipus and Apus. It follows, therefore, that for the following up of this clue, Limulus especially must be taken into consideration, while Branchipus and Apus are always to be kept in mind.
Ammocœtes rather than Amphioxus is the Best Subject for Investigation.
It is not, however, Limulus that must be investigated in the first instance, but the vertebrate itself; for it can never be insisted on too often that in the vertebrate itself its past history will be found, but that Limulus cannot reveal the future of its race. What vertebrate must be chosen for investigation? Reasons have been given why our attention should be fixed upon the king-crab rather than on the lobster on the invertebrate side; what is the most likely animal on the vertebrate side?
From the evidence already given it is manifest that the earliest mammal belonged to the lowest group of mammals; that the birds on their first appearance presented reptilian characteristics, that the earliest reptiles belonged to a low type of reptile, that the amphibians at their first appearance were nearer in type to the fishes than were the later forms. As each of these groups advances in number and power, specialization takes place in it, and the latest developed members become further and further removed in type from the earliest. So also it must have been with the origin of fishes: here too, in the quest for information as to the structure and nature of the first-formed fishes, we must look to the lowest rather than to the highest living members of the group.
The lowest fish-like animal at present living is Amphioxus, and on this ground it is argued that the original vertebrate must have approached in organization to that of Amphioxus; it is upon the comparison between the structure of Amphioxus and that of Balanoglossus, that the theory of the origin of vertebrates from forms like the latter animal is based. For my own part, I think that in the first instance, at all events, Amphioxus should be put on one side, although of course its structure must always be kept in mind, for the following reasons:—
Amphioxus, like the tunicates, does not possess the characteristics of other vertebrates. In all vertebrates above these forms the great characteristic is a well-defined brain-region from which arise nerves to organs of special sense, the eyes and nose. In Amphioxus no eyes exist, for the pigmented spot at the anterior extremity of the brain-region is no eye but only a mass of pigment, and the so-called olfactory pit is a very rudimentary and inferior organ of smell. In connection with the nearly complete absence of these two most important sense-organs, the most important part of the central nervous system, the region corresponding to the cerebral hemispheres, is also nearly completely absent.
Now, the history of the evolution of the central nervous system in the animal race points directly to its formation as a concentrated mass of nervous material at the anterior extremity of the body, in consequence of the formation of special olfactory and visual organs at that extremity. As already stated, the concentration of nervous material around the mouth as an oral ring was its beginning. In connection with this there arose special sense-organs for the guidance of the animal to its food which took the form of olfactory and optic organs. With the shifting from the radial to the elongated form these sense-organs remained at the anterior or mouth-end of the animal, and owing to their immense importance in the struggle for existence, that part of the central nervous system with which they were connected developed more than any other part, became the leader to which the rest of the nervous system was subservient, and from that time onwards the development of the brain-region was inevitably associated with the upward progress of animal life.
To those who believe in Evolution and the Darwinian theory of the survival of the fittest, it is simply inconceivable that a soft-bodied animal living in the mud, blind, with a rudimentary brain and rudimentary olfactory organs, such as is postulated when we think of Balanoglossus and Amphioxus, should hold its own and come victorious out of the struggle for existence at a time when the sea was peopled with powerful predaceous scorpion- and crab-like armour-plated animals possessing a well-developed brain, good eyes and olfactory organs, and powerful means of locomotion. Wherever in the scale of animal development Amphioxus may ultimately be placed, it cannot be looked upon as the type of the earliest formed fishes such as appeared in Silurian times.
The next lowest group of living fishes is the Marsipobranchii which include the lampreys and hag-fishes. To these naturally we must turn for a clue as to the organization of the earliest fish, for here we find all the characteristics of the vertebrates represented: a well-formed brain-region, well-developed eyes and nose, cranial nerves directly comparable with those of other vertebrates, and even the commencement of vertebræ.
Among these forms the lamprey is by far the best for investigation, not only because it is easily obtainable in large quantities, but especially because it passes a large portion of its existence in a larval condition, from which it emerges into the adult state by a wonderful process of transformation, comparable in extent with the transformation of the larval caterpillar into the adult imago. So long does the lamprey live in this free larval condition, and so different is it in the adult stage, that the older anatomists considered that the two states were really different species, and gave the name of Ammocœtes branchialis to the larval stage, while the adult form was called Petromyzon planeri, or Petromyzon fluviatilis.
This long-continued free-living existence in the larval or Ammocœtes stage makes the lamprey, more than any other type of lowly organized fish, invaluable for the present investigation, for throughout the animal kingdom it is recognized that the larval form approaches nearer to the ancestral type than the adult form, whether the latter is progressive or degenerate. Not only are the tissues formed during the stages which are passed through in a free-living larval form, serviceable tissues comparable to those of adult life, but also these stages proceed at so much slower a rate than do those in the embryo in utero or in the egg, as to make the larval form much more suitable than the embryo for the investigation of ancestral problems. It is true enough that the free life of the larva may bring about special adaptations which are not of an ancestral character, as may also occur during the life of the adult; but the evidence is very strong that although some of the peculiarities of the larva may be due to such cœnogenetic factors, yet on the whole many of them are due to ancestral characters, which disappear when transformation takes place, and are not found in the adult.
Thus if it be supposed that the amphibian arose from the fish, the tadpole presents more resemblance to the fish than the frog. If it be supposed that the arthropod arose from the segmented worm, the caterpillar bears out the suggestion better than the adult imago. If it be supposed that the tunicate arose from a stock allied to the vertebrate, it is because of the peculiarities of the larva that such a supposition is entertained. So, too, if it be supposed that the fish arose from a member of the arthropod group, the larval form of the fish is most likely to give decisive information on the point.
For all these reasons the lowest form of fish to be investigated, in the hopes of finding out the nature of the earliest formed fish, is not Amphioxus, but Ammocœtes, the larval form of the lamprey—a form which, as I hope to satisfy my reader after perusal of subsequent pages, more nearly resembles the ancient Cephalaspidian fishes than any other living vertebrate.
Comparison of Central Nervous Systems of Vertebrate and Arthropod without Reversal of Surfaces.
So far different lines of investigation all point to the origin of the vertebrate from arthropods, the group of arthropods in question being now extinct, the nearest living representative being Limulus; also to the fact that of the two theories of the origin of vertebrates, that one which is based on the resemblance between the central nervous systems of the Vertebrata and the Appendiculata (Arthropoda and Annelida) is more in accordance with this evidence than the other, which is based mainly on the supposed possession of a notochord among certain animals.
How is it, then, that this theory has been discredited and lost ground? Simply, I imagine, because it was thought to necessitate the turning over of the animal. Let us, then, again look at the nervous system of the vertebrate, and see whether there is any such necessity.
As previously mentioned, the comparison of the two central nervous systems showed such close resemblances as to force those anatomists who supported this theory to the conclusion that the infundibular tube was in the position of the original œsophagus; they therefore looked for the remains of a mouth opening in the dorsal roof of the brain, but did not attempt to explain the extraordinary fact that the infundibular tube is only a ventral offshoot from the tube of the central nervous system. Yet this latter tube is one, if not the most striking, of the peculiarities which distinguish the vertebrate; a tubular central nervous system such as that of the vertebrate is totally unlike any other nervous system, and the very fact that the two nervous systems of the vertebrate and arthropod are so similar in their nervous arrangements, makes it still more extraordinary that the nervous system should be grouped round a tube in the one case and not in the other.
Now, in the arthropod the œsophagus leads directly into the stomach, which is situated in the head-region, and from this a straight intestine passes directly along the length of the body to the anus, where it terminates. The relations of mouth, œsophagus, alimentary canal, and nervous system in these animals are represented in the diagram (Fig. 3).
Any tube, therefore, such as that of the infundibulum, which would represent the œsophagus of such an animal, must have opened into the mouth on the ventral side, and into the stomach on the dorsal side, and the lining epithelium of such an œsophagus must have been continuous with that of the stomach, and so of the whole intestinal tract.
Supposing, then, the animal is not turned over, but that the dorsal side still remains dorsal and ventral ventral, then the original mouth-opening of the œsophagus must be looked for on the ventral surface of the vertebrate brain in the region of the pituitary body or hypophysis, and on the dorsal side the tube representing the œsophagus must be continuous with a large cephalically dilated tube, which ought to pass into a small canal, to run along the length of the body and terminate in the anus.
This is exactly what is found in the vertebrate, for the infundibular tube passes into the third ventricle of the brain, which forms, with the other ventricles of the brain, the large dilated cephalic portion of the so-called nerve tube, and at the junction of the medulla oblongata and spinal cord, this dilated anterior part passes into the small, straight, central canal of the spinal cord, which in the embryo terminates in the anus by way of the neurenteric canal. If the animal is regarded as not having been turned over, then the conclusion that the infundibulum was the original œsophagus leads immediately to the further conclusion that the ventricles of the vertebrate brain represent the original cephalic stomach, and the central canal of the spinal cord the straight intestine of the arthropod ancestor.
For the first time a logical, straightforward explanation is thus given of the peculiarities of the tube of the central nervous system, with its extraordinary termination in the anus in the embryo, its smallness in the spinal cord, its largeness in the brain region, and its offshoot to the ventral side of the brain as the infundibular channel. It is so clear that, if the infundibular tube be looked on as the old œsophagus, then its lining epithelium is the lining of that œsophagus; and the fact that this lining epithelium is continuous with that of the third ventricle, and so with the lining of the whole nerve-tube, must be taken into account and not entirely ignored as has hitherto been the case. If, then, we look at the central nervous system of the vertebrate in the light of the central nervous system of the arthropod without turning the animal over, we are led immediately to the conclusion that what has hitherto been called the vertebrate nervous system is in reality composed of two parts, viz. a nervous part comparable in all respects with that of the arthropod ancestor, which has grown over and included into itself, to a greater or less extent, a tubular part comparable in all respects with the alimentary canal of the aforesaid ancestor. If this conclusion is correct, it is entirely wrong to speak of the vertebrate central nervous system as being tubular, for the tube does not belong to the nervous system, but was originally a simple epithelial tube, such as characterizes the œsophagus, cephalic stomach, and straight intestine of the arthropod.
Here, then, is the crux of the position—either the so-called nervous tube of the vertebrate is composed of two separate factors, consisting of a true non-tubular nervous system and a non-nervous epithelial tube, these two elements having become closely connected together; or it is composed of one factor, an epithelial tube which constitutes the nervous system, its elements being all nervous elements.
If this latter hypothesis be accepted, then it is necessary to explain why parts of that tube, such as the roof of the fourth ventricle, the choroid plexuses of the various ventricles, which are parts of the original roof inserted into the ventricles, are not composed of nervous material, but form simple single-layered epithelial sheets, which by no possibility can be included among functional nervous structures. The upholders of this hypothesis can only explain the nature of these thin epithelial parts of the nervous tube in one of two ways; either the tube was originally formed of nervous material throughout, and for some reason parts of it have lost their nervous function and thinned down; or else these thin epithelial parts are on their way to become nervous material, are still in an embryonic condition, and are of the nature of epiblast-epithelium, from which the central nervous system originally arose.
The first explanation is said to be supported by embryology, for at first the nerve-tube is formed in a uniform manner, and then later, parts of the roof appear to thin out and so form the thin epithelial parts. If this were the right explanation, then it ought to be found that in the lowest vertebrates there is greater evidence of a uniformly nervous tube than in the higher members of the group: while conversely, if, on the contrary, as we descend the vertebrate phylum, it is found that more and more of the tube presents the appearance of a single layer of epithelium, and the nervous material is limited more and more to certain parts of that tube, then the evidence is strong that the tubular character of the central nervous system is not due to an original nervous tube, but to a non-nervous epithelial tube with which the original nervous system has become closely connected.
The comparison of the brain region of the different groups of vertebrates (Fig. 19) is most instructive, for it demonstrates in the most conclusive manner how the roof of the nervous tube in that region loses more and more its nervous character, and takes on the appearance of a simple epithelial tube, as we descend lower and lower; until at last, in the brain of Ammocœtes, as represented in the figures, the whole of the brain-roof, from the region of the pineal eye to the commencement of the spinal cord, is composed of fold upon fold of a thin epithelial membrane forming an epithelial bag, which is constricted in only one place, where the fourth cranial nerve crosses over it.
Further, the brain of Ammocœtes (Fig. 20) shows clearly not only that it is composed of two parts, an epithelial tube and a nervous system, but also that the nerve-masses are arranged in the same relative position with respect to this tube as are the nerve-masses in the invertebrate with respect to the cephalic stomach and œsophagus. This evidence is so striking, so conclusive, that it is impossible to resist the conclusion that the tube did not originate as part of the central nervous system, but was originally independent of the central nervous system, and has been invaded by it.
Fig. 19.—Comparison of Vertebrate Brains.
CB., cerebellum; PT., pituitary body; PN., pineal body; C. STR., corpus striatum; G.H.R., right ganglion habenulæ. I., olfactory; II., optic nerves.
Fig. 20.—Brain of Ammocœtes.
The second explanation is hardly worth serious consideration, for it supposes that the nervous system, for no possible reason, was laid down in its most important parts—the brain-region—as an epithelial tube with latent potential nervous functions; that even up to the highest vertebrate yet evolved these nervous functions are still in abeyance over the whole of the choroid plexuses and the roof of the fourth ventricle. Further, it supposes that this prophetic epithelial tube originally developed into true nervous material only in certain parts, and that these parts, curiously enough, formed a nervous system absolutely comparable to that of the arthropod, while the dormant prophetic epithelial part was formed so as just to mimic, in relation to the nervous part, the alimentary canal of that same arthropod.
The mere facts of the case are sufficient to show the glaring absurdity of such an explanation. This is not the way Nature works; it is not consistent with natural selection to suppose that in a low form nervous material can be laid down as non-nervous epithelial material in order to provide in some future ages for the great increase in the nervous system.
Every method of investigation points to the same conclusion, whether the method is embryological, anatomical, or pathological.
First, take the embryological evidence. On the ground that the individual development reproduces to a certain extent the phylogenetic development, the peculiarities of the formation of the central nervous system in the vertebrate embryo ought to receive an appropriate explanation in any theory of phylogenetic development. Hitherto such explanation has been totally lacking; any suggestion of the manner in which a tubular nervous system may have been formed takes no account whatever of the differences between different parts of the tube; its dilated cephalic end with its infundibular projection ventrally, its small straight spinal part, and its termination in the anus. My theory, on the other hand, is in perfect harmony with the embryological history, and explains it point by point.
From the very first origin of the central nervous system there is evidence of two structures—the one nervous, and the other an epithelial surface-layer which ultimately forms a tube; this was first described by Scott in Petromyzon, and later by Assheton in the frog. In the latter case the external epithelial layer is pigmented, while the underlying nervous layer contains no pigment; a marked and conspicuous demarcation exists, therefore, between the two layers from the very beginning, and it is easy to trace the subsequent fate of the two layers owing to this difference of pigmentation. The pigmented cells form the lining cells of the central canal, and becoming elongated, stretch out between the cells of the nervous layer; while the latter, on their side, invade and press between the pigmented cells. In this case, owing to the pigmentation of the epithelial layer, embryology points out in the clearest possible manner how the central nervous system of the vertebrate is composed of two structures—an epithelial non-nervous tube, on the outside of which the central nervous system was originally grouped; how, as development proceeds, the elements of these two structures invade each other, until at last they become so involved together as to give rise to the conception that we are dealing with one single nerve tube. It is impossible for embryology to give a clearer clue to the past history than it does in this case, for it actually shows, step by step, how the amalgamation between the central nervous system and the old alimentary canal took place.
Further, consider the shape of the tube when it is first formed, how extraordinary and significant that is. It consists of a simple dilated anterior end leading into a straight tube, the lumen of which is much larger than that of the ultimate spinal canal, and terminates by way of the neurenteric canal in the anus.
Why should the tube take this peculiar shape at its first formation? No explanation is given or suggested in any text-book of embryology, and yet it is so natural, so simple: it is simply the shape of the invertebrate alimentary canal with its cephalic stomach and straight intestine ending in the anus. Again embryology indicates most unmistakably the past history of the race. How are the nervous elements grouped round this tube when it is first formed? Here embryology shows that a striking difference exists between the part of the tube which forms the spinal cord and the dilated cephalic part. Fig. 21, A (2), represents the relation between the nervous masses and the epithelial tube in the first instance. At this stage the nervous material in the spinal cord lies laterally and ventrally to this tube, and at a very early stage the white anterior commissure is formed, joining together these two lateral masses; as yet there is no sign of any posterior fissure, the tube with its open lumen extends right to the dorsal surface.
The interpretation of this stage is that in the invertebrate ancestor the nerve-masses were situated laterally and ventrally to the epithelial tube, and were connected together by commissures on the ventral side of the tube (Fig. 21, A (1)); in other words, the chain of ventral ganglia and their transverse commissures lying just ventrally to the intestine, which are so characteristic of the arthropod nervous system, is represented at this stage.
Subsequently, by the growth dorsalwards of nervous material to form the posterior columns, the original epithelial tube is compressed dorsally and laterally to such an extent that those parts lose all signs of lumen, the one becoming the posterior fissure and the others the substantia gelatinosa Rolandi on each side. The original tube is thus reduced to a small canal formed by its ventral portion only (Fig. 21, A (3)). In this way the spinal cord is formed, and the walls of the original epithelial tube are finally visible only as the lining of the central canal (Fig. 21, A (4)).
When we pass to the brain-region, to the anterior dilated portion of the tube, embryology tells a different story. Here, as in the spinal cord, the nervous masses are grouped at first laterally and ventrally to the epithelial tube, as is seen in Fig. 21, B (2), but owing to the large size of its lumen here, the nervous material is not able to enclose it completely, as in the case of the spinal cord; consequently there is no posterior fissure formed; but, on the contrary, the dorsal roof, not enclosed by the nerve-masses, remains epithelial, and so forms the membranous roof of the fourth ventricle and of the other ventricles of the brain (Fig. 21, B (3)). In the higher animals, owing to the development of the cerebrum and cerebellum, this membranous roof becomes pushed into the larger brain cavity, and thus forms the choroid plexuses of the third and lateral ventricles. In the lower vertebrates, as in Ammocœtes and the Dipnoi, it still remains as a dorsal epithelial roof and forms a most striking characteristic of such brains.
In this part of the nervous system, then, the nervous material is all grouped in its original position on the ventral side of the tube; and yet it is the same nervous material as that of the spinal cord, all the elements are there, giving origin here to the segmental cranial nerves just as lower down they give rise to the segmental spinal nerves, connecting together the separate segments each with the other and all with the higher brain-centres—the supra-infundibular centres—just as they do in the spinal region.
Why should there be this striking difference between the formation of the infra-infundibular region of the brain and that of the spinal cord? Do the advocates of the origin of vertebrates from Balanoglossus give the slightest reason for it? They claim that their view also provides a tubular nervous system for the vertebrate, but give not the slightest sign or indication as to why the nervous material should be grouped entirely on the ventral side of an epithelial tube in the infra-infundibular region and yet surround it in the spinal cord region. And the explanation is so natural, so simple: embryology does its very best to tell us the past history of the race, if only we look at it the right way.
The infra-infundibular nervous mass is naturally confined to the ventral side of the epithelial tube, because it represents the infra-œsophageal ganglia, situated as they are on the ventral side of the cephalic stomach, and, owing to the size of the stomach, they could not enclose it by dorsal growth, as they do in the case of the formation of the spinal cord (Fig. 21, B (1)). Still these nervous masses have grown dorsalwards, have commenced to involve the walls of the cephalic stomach even in the lowest vertebrate, as is seen in Ammocœtes, in which animal a ventral portion of the epithelial bag has been evidently compressed and its lumen finally obliterated by the growth of the nerve-masses on each side of it. Throughout the whole vertebrate kingdom this obliterated portion still leaves its mark as the raphé or seam, which is so characteristic of the infra-infundibular portion of the brain.
Fig. 22.—Horizontal Section through the Brain of Ammocœtes.
Here, again, it is seen how simple is the explanation of a peculiarity which has always puzzled anatomists—why should there be this seam in the infra-infundibular portion of the brain and not in the supra-infundibular or in the spinal cord? The corresponding compression in the upper brain-region forms the lateral ventricles, as is seen in the accompanying figure of the brain of Ammocœtes (Fig. 22).
Fig. 23.—Section through Rhomboidal Sinus of Bird.
In yet another instance it is seen how markedly the nervous masses are arranged in the same position with respect to the central tube as are the nerve ganglia with respect to the intestinal tube in the case of the invertebrate. Thus in birds a portion of the spinal cord in the lumbo-sacral region presents a very different appearance from the rest of the cord; it is known as the rhomboidal sinus, and a section of the cord of an adult pigeon across this region is given in Fig. 23. As is seen, the nervous portions are entirely confined to two masses connected together by the white anterior commissures which are situated laterally and ventrally to a median gelatinous mass; the small central canal is visible and the whole dorsal area of the cord is taken up by a peculiar non-nervous wedge-shaped mass of tissue. At its first formation this portion of the cord is formed exactly in the same manner as the rest of the cord; instead, however, of the nervous material invading the dorsal part of the tube to form the posterior fissure, it has been from some cause unable to do so, the walls of the original non-nervous tube have become thickened dorsally, been transformed into this peculiar tissue, and so caused the peculiar appearance of the cord here. The nervous parts have not suffered in their development; the mechanism for walking in the bird is as well developed as in any other animal; their position only is different, for they still retain the original ventro-lateral position, but the non-nervous tube, the remains of the old intestine, has undergone a peculiar gelatinous degeneration just where it has remained free from invasion by the nervous tissue.
Throughout the whole of that part of the nervous system which gives origin to the cranial and spinal segmental nerves, the evidence is absolutely uniform that the nervous material was originally arranged bilaterally and ventrally on each side of the central tube, exactly in the same way as the nerve-masses of the infra-œsophageal and ventral chain of ganglia are arranged with respect to the cephalic stomach and straight intestine of the arthropod. But, in addition, we find in the vertebrate nervous masses, the cerebral hemispheres, the corpora quadrigemina and the cerebellum situated on the dorsal side of the central tube in the brain-region; this nervous material is, however, of a different character to that which gives origin to the spinal and cranial segmental nerves. How is the presence of these dorsal masses to be explained on the supposition that the dilated anterior part of the nerve-tube was originally the cephalic stomach of the arthropod ancestor? The cerebral hemispheres are simple enough, for they represent the supra-œsophageal ganglia, which of necessity, as they increased in size, would grow round the anterior end of the cephalic stomach and become more and more dorsal in position.
The difficulty lies rather in the position of the cerebellum and corpora quadrigemina, and the solution is as simple as it is conclusive.
Let us again turn to embryology and see what help it gives. In all vertebrates the dilated anterior portion of the nerve-tube does not, as it grows, increase in size uniformly, but a constriction appears on its dorsal surface at one particular place, so as to divide it into an anterior and posterior vesicle; then the latter becomes divided into two portions by a second constriction. In this way three cerebral vesicles are formed; these three primary cerebral vesicles indicate the region of the fore-brain, mid-brain, and hind-brain respectively. Subsequently the first cerebral vesicle becomes divided into two to form the prosencephalon and thalamencephalon, while the third cerebral vesicle is also divided into two to form the region of the cerebellum and medulla oblongata.
These constrictions are in the position of commissural bands of nervous matter; of these the limiting nervous strands between the thalamencephalon and mesencephalon and between the mesencephalon and the hind-brain are of primary importance. The first of these commissural bands is in the position of the posterior commissure connecting the two optic thalami. In close connection with this are found, on the mid-dorsal region, the two pineal eyes with their optic ganglia, the so-called ganglia habenulæ. From these ganglia a peculiar tract of fibre, known as Meynert's bundle, passes on each side to the ventral infra-infundibular portion of the brain. In other words, the first constriction of the dilated tube is due to the presence and growth of nervous material in connection with the median pineal eyes. Here in precisely the same spot, as will be fully explained in the next chapter, there existed in the arthropod ancestor a pair of median eyes situated dorsally to the cephalic stomach, the pre-existence of which explains the reason for the first constriction.
The second primary constriction separating the mid-brain from the hind-brain is still more interesting, for it is coincident with the position of the trochlear or fourth cranial nerve. In all vertebrates without exception this nerve takes an extraordinary course; all other nerves, whether cranial or spinal, pass ventralwards to reach their destination. This nerve passes dorsalwards, crosses its fellow mid-dorsally in the valve of Vieussens, where the roof of the brain is thin, and then passes out to supply the superior oblique muscle of the eye of the opposite side. The two nerves form an arch constricting the dilated tube at this place. In the lowest vertebrate (Ammocœtes) the constriction formed by this nerve-pair is evident not only in the embryonic condition as in other vertebrates, but during the whole larval stage. As Fig. 20, A and B, shows, the whole of the dorsal region of the brain up to the region of the pineal eye and ganglion habenulæ is one large membranous bag, except for the single constriction where the fourth nerve on each side crosses over. The explanation of this peculiarity is given in Chapter VII., and follows simply from the facts of the arrangement of that musculature in the scorpion-group which gave rise to the eye-muscles of the vertebrate.
In Ammocœtes both cerebellum and posterior corpora quadrigemina can hardly be said to exist, but upon transformation a growth of nervous material takes place in this region, and it is seen that this commencing cerebellum and the corpora quadrigemina arise from tissue that is present in Ammocœtes along the course of the fourth nerve.
Here, then, again Embryology does its best to tell us how the vertebrate arose. The formation of the two primary constrictions in the dilated anterior vesicle whereby the brain is divided into fore-brain, mid-brain, and hind-brain is simply the representation ontogenetically of the two nerve-tracts which crossed over the cephalic stomach in the prevertebrate stage, in consequence of the mid-dorsal position of the pineal eyes and of the insertion of the original superior oblique muscles.
The subsequent constriction by which the prosencephalon is separated from the thalamencephalon is in the position of the anterior commissure, that commissure which connects the two supra-infundibular nerve-masses, and is one of the first-formed commissures in every vertebrate. This naturally is simply the commissure between the two supra-œsophageal ganglia; anterior to it, in the middle line, equally naturally, the anterior end of the old stomach wall still exists as the lamina terminalis.
The other division in the hind-brain region, which separates the region of the cerebellum from the medulla oblongata, is due to the growth of the cerebellum, and indicates its posterior limit. In such an animal as the lamprey, where the cerebellum is only commencing, this constriction does not occur in the embryo.
From such simple beginnings as are seen in Ammocœtes, the higher forms of brain have been evolved, to culminate in that of man, in which the massive cerebrum and cerebellum conceals all sign of the dorsal membranous roof, those parts of the simple epithelial tube which still remain being tucked away into the cavities to form the various choroid plexuses.
In the whole evolution from the brain of Ammocœtes to that of man, the same process is plainly visible, viz. growth and extension of nervous material over the epithelial tube; extension dorsally and posteriorly of the supra-infundibular nervous masses (as seen in Fig. 19), combined with a dorsal growth of parts of the infra-infundibular nervous masses to form the cerebellum and posterior corpora quadrigemina.
Especially instructive is the formation of the cerebellum. It consists at first of a small mass of nervous tissue accompanying the fourth nerve, then by the growth of that mass surrounding and constricting a fold of the membranous roof, the worm of the cerebellum is formed, as in the dog-fish. This very constriction causes the membrane to be thrown into a lateral fold on each side, as seen in Fig. 24, and in the dog-fish the nervous material on each side, known as the fimbriæ, is already commencing to grow from the ventral mass of the medulla oblongata to surround these lateral membranous folds. These fimbriæ develop more and more in higher forms, and thus form the cerebellar hemispheres.
Not only does comparative anatomy confirm the teachings of embryology, but also pathology gives its quota in the same direction.
Fig. 24.—Cerebellum of Dog-fish.
One of the striking facts about malformations and disease of the central nervous system is the frequency of cystic formations; spina bifida is a well-known instance. These cysts are merely epithelial non-nervous cysts formed from the epithelium of the central canal, difficult to understand if the whole nerve tube is one and entirely nervous, either actually or potentially, but natural and easy if we are really dealing with a simple epithelial tube on the outside of which the nervous material was originally grouped. The cystic formation belongs naturally enough to this tube, not to the nervous system.
Again, where animals such as lizards have grown a new tail, owing to the breaking off of the original one, it is found that the central canal extends into this new tail for some distance, but not the nervous material surrounding it; all the nerves supplying the new tail arise from the uninjured spinal cord above, the central canal with its lining layer of epithelial cells alone grows into the new-formed appendage.
To all intents and purposes the same thing is seen in the termination of the spinal cord in a bird-embryo; more and more, as the end of the tail is approached, does the nervous matter of the spinal cord grow less and less, until at last a naked central canal with its lining epithelium is alone left to represent the so-called nerve-tube.
All these different methods of investigation lead irresistibly to the one conclusion that the tubular nature of the central nervous system has been caused by the central nervous system enclosing to a greater or less extent a pre-existing, non-nervous, epithelial tube.
This must always be borne strictly in mind. The problem, therefore, which presents itself is the comparison of these two factors separately, in order to find out the relationship of the vertebrate to the invertebrate. The nervous system without the tube must be compared to other nervous systems, and the tube must be considered apart from the nervous system.
The Principle of Concentration and Cephalization.
The central nervous system of the vertebrate resembles that of all the Appendiculata in the fact that it is composed of segments joined together which give origin to segmental nerves. There is, however, a great difference between the two systems: the division into separate segments is not obvious to the eye in the vertebrate nervous system, while in the invertebrate we can see that it is composed of a series of separate pairs of ganglia joined together longitudinally by nervous strands known as connectives and transversely by the nerve-commissures. Such a simple segmented system is found in the segmented worms, and in the lower arthropods, such as Branchipus, no great advance has been made on that of the annelid. In the higher forms, however, a greater and greater tendency to fusion of separate ganglia exists, especially in the head-region, so that the infra-œsophageal ganglia, which, in the lower forms are as separate as those of the ventral chain, in the higher forms are fused together to form a single nervous mass.
This is the great characteristic of the advancement of the central nervous system among the Invertebrata, its concentration in the region of the head. It may be called the principle of cephalization, and is characteristic not only of higher organization in a group, but also of the adult as distinguished from the larval form. Thus in the imago greater concentration is found than in the caterpillar.
The segmented annelid type of nervous system consists of a supra-œsophageal ganglion, composed of the fused ganglia belonging to the pre-oral segments, and an infra-œsophageal chain of separate ganglia. With the concentration and modification around the mouth of the most anterior locomotor appendages to form organs for prehension and mastication of food, a corresponding concentration and fusion of the ganglia belonging to these segments takes place, so that finally, in the higher annelids, and in most of the great arthropod group, a fusion of a number of the most anterior ganglia has taken place to form the infra-œsophageal ganglion-mass.
The infra-œsophageal ganglia which are the first to fuse are those which supply the most anterior portion of the animal with nerves, and include always those anterior appendages which are modified for mastication purposes. To this part the name prosoma has been given; in many cases it forms a well-defined, distinct portion of the animal.
Succeeding this prosoma or masticatory region, there occurs in all gill-bearing arthropods a respiratory region, in many cases more or less distinctly defined, which has received the name of mesosoma. The rest of the body is called the metasoma.
In accordance with this nomenclature the central nervous system of many of the Arthropoda may be divided as follows:—
1. Pre-oral, or supra-œsophageal ganglia.
2. Infra-oral, or infra-œsophageal ganglia and ventral chain, which consist of three groups: prosomatic, mesosomatic, and metasomatic ganglia.
The infra-œsophageal ganglion-mass, then, in most of the Arthropoda may be spoken of as formed by the fusion of the prosomatic or mouth-ganglia, the mesosomatic and metasomatic remaining separate and distinct. The number of ganglia which have fused may be observed by examination of the embryo, in which it is easy to see indications of the individual ganglia or neuromeres, although all such indication has disappeared in the adult; thus the infra-œsophageal ganglia of the cray-fish have been shown to be constituted of six prosomatic ganglia.
In Fig. 25 I give figures of the central nervous system (with the exception of the abdominal or metasomatic ganglia) of Branchipus, Astacus, Limulus, Scorpio, Androctonus, Thelyphonus, and Ammocœtes. In all the figures the supra-œsophageal ganglia are lined horizontally, and their nerves shown, viz. optic (lateral eyes (II) and median eyes (II´)), olfactory (I) (first antennæ, camerostome, nose); then come the prosomatic ganglia (dotted), with their nerves (A) supplying the mouth parts, and the second antennæ or cheliceræ; then the mesosomatic (lined horizontally), with their nerves (B) supplying respiratory appendages. These figures show that the concentrated brain mass around the œsophagus of an arthropod which has arrived at the stage of Astacus, is represented by the supra-œsophageal ganglia and the fused prosomatic ganglia.
The next stage in the evolution of the brain is seen in the gradual inclusion of the mesosomatic ganglia, one after the other, into the infra-œsophageal mass of the already fused prosomatic ganglia. With this fusion is associated the loss of locomotion in these mesosomatic appendages, and their entire subservience to the function of respiration. Dana urges that cephalization is a consequence of functional alteration in the appendages, from organs of locomotion to those of mastication and respiration. Whether this be true or not, it is certainly a fact that in Limulus, the ganglion supplying the first mesosomatic appendage has fused with the prosomatic, infra-œsophageal mass. It is also a fact that the prosomatic appendages are the organs of mastication, their basal parts being arranged round the mouth so as to act as foot-jaws, while the mesosomatic appendages, though still free to move, have been reduced to such an extent as to consist mainly of their basal parts, which are all respiratory in function, except in the case of the first pair, where they carry the terminal ducts of the genital organs. In the next stage, that, of the scorpion, in which the mesosomatic appendages have lost all power of free locomotion, and have become internal branchiæ, another mesosomatic ganglion has fused with the brain mass, while in Androctonus two of the branchial mesosomatic ganglia have fused; and finally, in Thelyphonus and Phrynus, all the mesosomatic ganglia have coalesced with the fused prosomatic ganglia, while the metasomatic ganglia have themselves fused together in the caudal region to form what is known as the caudal brain.
Fig. 25.—Comparison of Invertebrate Brains from Branchipus to Ammocœtes.
The brain in these animals may be spoken of as composed of three parts—(1) the fused supra-œsophageal ganglia, (2) the fused prosomatic ganglia, and (3) the fused mesosomatic ganglia. Such a brain is strictly homologous with the vertebrate brain, which also is built up of three parts—(1) the part in front of the notochord, the prechordal or supra-infundibular brain, which consists of the cerebral hemispheres, together with the basal and optic ganglia and corresponds, therefore, to the supra-œsophageal mass, with its olfactory and optic divisions lying in front of the œsophagus; (2 and 3) the epichordal brain, composed of (2) a trigeminal and (3) a vagus division, of which the first corresponds strictly to the fused prosomatic ganglia, and the second to the fused mesosomatic ganglia. Further, just as in the embryo of an arthropod it is possible, with more or less accuracy, to see the number of neuromeres or original ganglia which have fused to form the supra- and infra-œsophageal portions of its brain, so also in the embryo of a vertebrate we are able at an early stage to gain an indication, more or less accurate, of the number of neuromeres which have built up the vertebrate brain. The further consideration of these neuromeres, and the evidence they afford as to the number of the prosomatic and mesosomatic ganglia which have formed the epichordal part of the vertebrate brain, must be left to the chapter on the segmentation of the cranial nerves.
The further continuation of this process of concentration of separate segments, together with the fusion of the nervous system with the tube of the alimentary canal, leads in the simplest manner to the formation of the spinal cord of the vertebrate from the metasomatic ganglia of the ventral chain of the arthropod.
The Antagonism between Cephalization and Alimentation.
This concentration of the nervous system in the head-region, together with an actual increase in the bulk of the cephalic nervous masses, constitutes the great principle upon which the law of upward progress or evolution in the animal kingdom is based, and it illustrates in a striking manner the blind way in which natural selection works; for, as already explained, the central nervous system arose as a ring round the mouth, in consequence of which, with the progressive evolution of the animal kingdom, the œsophagus necessarily pierced the central nervous system at the cephalic end. At the same time, the very fact that the evolution was progressive necessitated the concentration and increase of the nervous masses in this very same œsophageal region.
Progress on these lines must result in a crisis, owing to the inevitable squeezing out of the food-channel by the increasing nerve-mass; and, indeed, the fact that such a crisis had in all probability arisen at the time when vertebrates first appeared is apparent when we examine the conditions at the present time.
Those invertebrates whose central nervous system is most concentrated at the cephalic end belong to the arachnid group, among which are included the various living scorpion-like animals, such as Thelyphonus, Androctonus, etc.
As already mentioned, the giants of the Palæostracan age were Pterygotus, Slimonia, etc., all animals of the scorpion-type—in fact, sea-scorpions. Now, all these animals, spiders and scorpions, without exception, are blood-suckers, and in all of them the concentrated cephalic mass of nervous material surrounds an œsophagus the calibre of which is so small that nothing but a fluid pabulum can be taken into the alimentary canal; and even for that purpose a special suctorial apparatus has in some species been formed on the gastric side of the œsophagus for the purpose of drawing blood through this exceedingly narrow tube.
In Fig. 25 this increasing antagonism between brain-power and alimentation, as we pass from such a form as Branchipus to the scorpion, is illustrated, and in Fig. 26 the relative sizes of the œsophagus and the brain-mass surrounding it is shown. The section shows that the food channel is surrounded by the white and grey matter of the brain as completely as the central canal of the spinal cord of the vertebrate is surrounded by the white and grey nervous material.
Fig. 26.—Transverse Section through the Brain of a Young Thelyphonus.
A, supra-œsophageal ganglia; B, infra-œsophageal ganglia; Al, œsophagus.
Truly, at the time when vertebrates first appeared, the direction and progress of variation in the Arthropoda was leading, owing to the manner in which the brain was pierced by the œsophagus, to a terrible dilemma—either the capacity for taking in food without sufficient intelligence to capture it, or intelligence sufficient to capture food and no power to consume it.
Something had to be done—some way had to be found out of this difficulty. The atrophy of the brain meant degeneration and the reduction to a lower stage of organization, as is seen in the Tunicata. The further development of the brain necessitated the establishment of a new method of alimentation and the closure of the old œsophagus, its vestiges still remaining as the infundibular canal of the vertebrate, meant the enormous upward stride of the formation of the vertebrate.
At first sight it might appear too great an assumption even to imagine the possibility of the formation of a new gut in an animal so highly organized as an arthropod, but a little consideration will, I think, show that such is not the case.
In the higher animals we are accustomed to speak of certain organs as vital and necessary for the further existence of the animal; these are essentially the central nervous system, the respiratory system, the circulatory system, and the digestive system. Of these four vital systems the first cannot be touched without the chance of degeneration; but that is not the case with the second. The passage from the fish to the amphibian, from the water-breathing to the air-breathing animal, has actually taken place, and was effected by the modification of the swim-bladder to form new respiratory organs—the lung; the old respiratory organs—the gills—becoming functionless, but still persisting in the embryo as vestiges. The necessity arose in consequence of the passage of the animal from water to land, and with this necessity nature found a means of overcoming the difficulty; air-breathing vertebrates arose, and from the very fact of their being able to extend over the land-surfaces, increased in numbers and developed in complexity in the manner already sketched out.
For a respiratory system all that is required is an arrangement by means of which blood should be brought to the surface, so as to interchange its gases with those of the external medium; and it is significant to find that of all vertebrates the Amphibia alone are capable of an effective respiration by means of the skin.
As to the circulatory system, it is exceedingly easily modified. An animal such as Amphioxus has no heart; in some the heart is systemic, in others branchial; in some there are more than one heart; in others there are contractile veins in addition to a heart. There is no difficulty here in altering and modifying the system according to the needs of the individual.
For a digestive system all that is required is an arrangement for the digestion and absorption of food, a mechanism which can arise easily if some of the cells of the skin possess digestive power. Now Miss Alcock has shown that some of the surface-cells of crustaceans secrete a fluid which possesses digestive powers, and she has also shown that certain of the cells in the skin of Ammocœtes possess digestive power.
The difficulty, then, of forming a new digestive system in the passage from the arthropod to the vertebrate is very much the same as the difficulty in forming a new respiratory system in the passage from the water-breathing fish to the air-breathing amphibian—a change which does not strike us as inconceivable, because we know it has taken place.
The whole argument so far leads to the conclusion that vertebrates arose from ancient forms of arthropods by the formation of a new alimentary canal, and the enclosure of the old canal by the growing central nervous system. If this conclusion is true, then it follows that we possess a well-defined starting-point from which to compare the separate organs of the arthropod with those of the vertebrate, and if, in consequence of such working hypothesis, each organ of the arthropod is found in the vertebrate in a corresponding position and of similar structure, then the truth of the starting-point is proved as fully as can possibly be expected by deductive methods. It is, in fact, this method of comparative anatomy which has proved the descent of man from the ape, the frog from the fish, etc.
Let us, then, compare all the organs of such a low vertebrate as Ammocœtes with those of an arthropod of the ancient type.
Life History of the Lamprey—not a Degenerate Animal.
The striking peculiarity of the lamprey is its life-history. It lives in fresh water, spending a large portion of its life in the mud during the period of its larval existence: then comes a somewhat sudden transformation-stage, characterized, as in the lepidopterous larva, by a process of histolysis, by which many of the larval tissues are destroyed and new ones formed, with the result that the larval lamprey, or Ammocœtes, is transformed into the adult lamprey, or Petromyzon. This transformation takes place in August, at all events in the neighbourhood of Cambridge, and later in the year the transformed lamprey migrates to the sea, grows in size and maturity, and returns to the river the following spring up to its spawning beds, where it spawns and forthwith dies. How long it lives in the Ammocœtes stage is unknown; I myself have kept some without transformation for four years, and probably they live in the rivers longer than that before they change from their larval state. It is absolutely certain that very much the longest part of the animal's life is spent in the larval stage, and that with the maturity of the sexual organs and the production of the fertilized ova the life of the individual ends.
Now, the striking point of this transformation is that it produces an animal more nearly comparable with higher vertebrates than is the larval form; in other words, the transformation from larva to adult is in the direction of upward progress, not of degeneration. It is, therefore, inaccurate to speak of the adult lamprey as degenerate from a higher race of fishes represented by its larval form—Ammocœtes. Its transformation does not resemble that of the tunicates, but rather that of the frog, so that, just as in the case of the tadpole, the peculiarities of its larval form may be expected to afford valuable indications of its immediate ancestry. The very peculiarities to which attention must especially be paid are those discarded at transformation, and, as will be seen, these are essentially characteristic of the invertebrate and are not found in the higher vertebrates. In fact, the transformation of the lamprey from the Ammocœtes to the Petromyzon stage may be described as the casting off of many of its ancestral invertebrate characters and the putting on of the characteristics of the vertebrate type. It is this double individuality of the lamprey, together with its long-continued existence in the larval form, which makes Ammocœtes more valuable than any other living vertebrate for the study of the stock from which vertebrates sprang.
Many authorities hold the view that the lamprey, like Amphioxus, must be looked upon as degenerate, and therefore as no more suitable for the investigation of the problem of vertebrate ancestry than is Amphioxus itself. This charge of degeneracy is based on the statement that the lamprey is a parasite, and that the eyes in Ammocœtes are under the skin. The whole supposition of the degeneracy of the Cyclostomata arose because of the prevailing belief of the time that the earliest fishes were elasmobranchs, and therefore gnathostomatous. From such gnathostomatous fishes the cyclostomes were supposed to have descended, having lost their jaws and become suctorial in habit in consequence of their parasitism.
The charge of parasitism is brought against the lamprey because it is said to suck on to fishes and so obtain nutriment. It is, however, undoubtedly a free-swimming fish; and when we see it coming up the rivers in thousands to reach the spawning-beds, and sucking on to the stones on the way in order to anchor itself against the current, or holding on tightly during the actual process of spawning, it does not seem justifiable to base a charge of degeneration upon a parasitic habit, when such so-called habit simply consists in holding on to its prey until its desires are satisfied. If, of course, its suctorial mouth had arisen from an ancestral gnathostomatous mouth, then the argument would have more force.
Dohrn, however, gives absolutely no evidence of a former gnathostomatous condition either in Petromyzon or, in its larval state, Ammocœtes. He simply assumes that the Cyclostomata are degenerated fishes and then proceeds to point out the rudiments of skeleton, etc., which they still possess. Every point that Dohrn makes can be turned round; and, with more probability, it can be argued that the various structures are the commencement of the skeletal and other structures in the higher fishes, and not their degenerated remnants. Compare the life-history of the lamprey and of the tunicate. In the latter case we look upon the animal as a degenerate vertebrate, because the larval stage alone shows vertebrate characteristics; when transformation has taken place, and the adult form is reached, the vertebrate characteristics have vanished, and the animal, instead of reaching a higher grade, has sunk lower in the scale, the central nervous system especially having lost all resemblance to that of the vertebrate. In the former case a transformation also takes place, a marvellous transformation, characterized by two most striking facts. On the one hand, the resulting animal is more like a higher vertebrate, for, by the formation of new cartilages, its cranial skeleton is now comparable with that of the higher forms, and the beginnings of the spinal vertebræ appear; by the increased formation of nervous material, its brain increases in size and complexity, so as to compare more closely with higher vertebrate brains; its eyes become functional, and its branchiæ are so modified, simultaneously with the formation of the new alimentary canal in the cranial region, that they now surround branchial pouches which are directly comparable to those of higher vertebrates. On the other hand, the transformation process is equally characterized by the throwing off of tissues and organs, one and all of which are comparable in structure and function with corresponding structures in the Arthropoda—the thyroid of the Ammocœtes, the tentacles, the muco-cartilage, the tubular muscles, all these structures, so striking in the Ammocœtes stage, are got rid of at transformation. Here is the true clue. Here, in the throwing off of invertebrate characters, and the taking on of a higher vertebrate form, especially a higher brain, not a lower one, Petromyzon proclaims as clearly as is possible that it is not a degenerate elasmobranch, but that it has arisen from Ammocœtes-like ancestors, even though Myxine, Amphioxus, and the tunicates be all stages on the downward grade from those same Ammocœtes-like ancestors.
As to the eyes, they are functional in the adult form and as serviceable as in any fish. There is no sign of degeneracy; it is only possible to speak of a retarded development which lasts through the larval stage.
Comparison of Brain of Ammocœtes with that of an Arthropod.
Seeing that the steady progress of the development of the central nervous system is the most important factor in the evolution of animals, it follows that of all organs of the body, the central nervous system must be most easily comparable with that of the supposed ancestor. I will, therefore, start by comparing the brain of Ammocœtes with that of arthropods, especially of Limulus and of the scorpion-group.
The supra-infundibular portion of the brain in vertebrates corresponds clearly to the supra-œsophageal portion of the invertebrate brain in so far that in both cases here is the seat of the will. Voluntary action is as impossible to the arthropod deprived of its supra-œsophageal ganglia as to the vertebrate deprived of its cerebrum. It corresponds, also, in that from it arise the nerves of sight and smell and no other nerves; this is also the case with the supra-œsophageal ganglia, for from a portion of these ganglia arise the nerves to the eyes and the nerves to the first antennæ, of which the latter are olfactory in function. Thus, in the accompanying figure, taken from Bellonci, it is seen that the supra-œsophageal ganglia consist of a superior segment corresponding to the cerebrum, a middle segment from which arise the nerves to the lateral eyes and to the olfactory antennæ, corresponding to the basal ganglia of the brain and the optic lobes, and, according to Bellonci, of an inferior segment from which arise the nerves to the second pair of antennæ. This last segment is not supra-œsophageal in position, but is situated on the œsophageal commissures. It has been shown by Lankester and Brauer in Limulus and the scorpion to be in reality the first ganglion of the infra-œsophageal series, and not to belong to the supra-œsophageal group.
Fig. 27.—The Brain of Sphæroma serratum. (After Bellonci.)
Further, in Limulus, in the scorpion-group, and in all the extinct Eurypteridæ—in fact, in the Palæostraca generally—there are two median eyes in addition to the lateral eyes, which were innervated from these ganglia.
In Ammocœtes, then, if the supra-infundibular portion of the brain really corresponds to the supra-œsophageal of the palæostracan group, we ought to find, as indeed is the case, an optic apparatus consisting of two lateral eyes and two median eyes, innervated from the supra-infundibular brain-mass, and an olfactory apparatus built up on the same lines as in the scorpion-group, also innervated from this region. If, in addition, it be found that those two median eyes are degenerate eyes of the same type as the median eyes of Limulus and the scorpion-group, then the evidence is so strong as to amount to a proof of the correctness of the theory. This evidence is precisely what has been obtained in recent years, for the vertebrate did possess two median eyes in addition to the two lateral ones, and these two median eyes are degenerate eyes of the type found in the median eyes of arthropods and are not of the vertebrate type. Moreover, as ought also to be the case, they are most evident, and one of the pair is most nearly functional in the lowest perfect vertebrate, Ammocœtes.
Of all the discoveries made in recent years, the discovery that the pineal gland of the vertebrate brain was originally a pair of median eyes is by far the most important clue to the ancestry of the vertebrate, for not only do they correspond exactly in position with the median eyes of the invertebrates, but, being already degenerate and functionless in the lowest vertebrate, they must have been functional in a pre-vertebrate stage, thus giving the most direct clue possible to the nature of the pre-vertebrate stage. It is especially significant that in Limulus they are already partially degenerated. What, then, ought to be the structure and relation to the brain of the median and lateral eyes of the vertebrate if they originated from the corresponding organs of some one or other member of the palæostracan group?
This question will form the subject of the next chapter.
Summary.
The object of this book is to attempt to find out from what group of invertebrates the vertebrate arose; no attempt is made to speculate upon the causes of variation by means of which evolution takes place.
A review of the animal kingdom as a whole leads to the conclusion that the upward development of animals from an original cœlenterate stock, in which the central nervous system consists of a ring of nervous material surrounding the mouth, has led, in consequence of the elaboration of the central nervous system, to a general plan among the higher groups of invertebrates in the topographical arrangement of the important organs. The mouth is situated ventrally, and leads by means of the œsophagus into an alimentary canal which is situated dorsally to the central nervous system. Thus the œsophagus pierces the central nervous system and divides it into two parts, the supra-œsophageal ganglia and the infra-œsophageal ganglia. This is an almost universal plan among invertebrates, but apparently does not hold for vertebrates, for in them the central nervous system is always situated dorsally and the alimentary canal ventrally, and there is no piercing of the central nervous system by an œsophagus.
Yet a remarkable resemblance exists between the central nervous system of the vertebrate and that of the higher invertebrates, of so striking a character as to compel one school of anatomists to attempt the derivation of vertebrates from annelids. Now, the central nervous system of vertebrates forms a hollow tube, and a diverticulum of this hollow tube, known as the infundibulum, passes to the ventral surface of the brain in the very position where the œsophagus would have been if that brain had belonged to an annelid or an arthropod. This school of anatomists therefore concluded that this infundibular tube represented the original invertebrate œsophagus which had become closed and no longer opened into the alimentary canal owing to the formation of a new mouth in the vertebrate. As, however, the alimentary canal of the vertebrate is ventral to the central nervous system, and not dorsal, as in the invertebrate, it follows that the remains of the original invertebrate mouth into which the œsophagus (in the vertebrate the infundibular tube) must have opened must be searched for on the dorsal side of the vertebrate; and so the theory was put forward that the vertebrate had arisen from the annelid by the reversal of surfaces, the back of the one animal becoming the front of the other.
The difficulties in the way of accepting such reversal of surfaces have proved insuperable, and another school has arisen which suggests that evolution has throughout proceeded on two lines, the one forming groups of animals in which the central nervous system is pierced by the food-channel and the gut therefore lies dorsally to it, the other in which the central nervous system always lies dorsally to the alimentary canal and is not pierced by it. In both cases the highest products of the evolution have become markedly segmented animals, in the former, annelids and arthropods; in the latter, vertebrates. The only evidence on which such theory is based is the existence of low forms of animals, known as the Enteropneusta, the best known example of which is called Balanoglossus; they are looked upon as aberrant annelid forms by many observers.
This theory does not attempt to explain the peculiarities of the tube of the vertebrate central nervous system, or to account for the extraordinary resemblance between the structure and arrangement of the central nervous systems of vertebrates and of the highest invertebrate group.
Neither of these theories is satisfactory or has secured universal acceptance. The problem must be considered entirely anew. What are the guiding principles in this investigation?
The evolution of animal life on this earth can clearly, on the whole, be described as a process of upward progress culminating in the highest form—man; but it must always be remembered that whole groups of animals such as the Tunicata have been able to survive owing to a reverse process of degeneration.
If there is one organ more than another which increases in complexity as evolution proceeds, which is the most essential organ for upward progress, surely it is the central nervous system, especially that portion of it called the brain. This consideration points directly to the origin of vertebrates from the most highly organized invertebrate group—the Arthropoda—for among all the groups of animals living on the earth in the present day they alone possess a central nervous system closely comparable with that of vertebrates. Not only has an upward progress taken place in animals as a whole, but also in the tissues themselves a similar evolution is apparent, and the evidence shows that the vertebrate tissues resemble more closely those of the arthropod than of any other invertebrate group.
The evidence of geology points to the same conclusion, for the evidence of the rocks shows that before the highest mammal—man—appeared, the dominant race was the mammalian quadruped, from whom the highest mammal of all—man—sprung; then comes, in Mesozoic times, the age of reptiles which were dominant when the mammal arose from them. Preceding this era we find in Carboniferous times that the amphibian was dominant, and from them the next higher group—the reptiles—arose. Below the Carboniferous come the Devonian strata with their evidence of the dominance of the fish, from whom the amphibian was directly evolved. The evidence is so clear that each succeeding higher form of vertebrate arose from the highest stage reached at the time, as to compel one to the conclusion that the fishes arose from the race which was dominant at the time when the fishes first appeared. This brings us to the Silurian age, in which the evidence of the rocks points unmistakably to the sea-scorpions, king-crabs, and trilobites as being the dominant race. It was preceded by the great trilobite age, and the whole period, from the first appearance of the trilobite to the time of dwindling away of the sea-scorpions, may be designated the Palæostracan age, using the term Palæostraca to include both trilobites and the higher scorpion and king-crab forms evolved from them. The evidence of geology then points directly and strongly to the origin of vertebrates from the Palæostraca—arthropod forms which were not crustacean and not arachnid, but gave origin both to the modern-day crustaceans and arachnids. The history of the rocks further shows that these ancient fishes, when they first appeared, resembled in a remarkable manner members of the palæostracan group, so that again and again palæontologists have found great difficulty in determining whether a fossil is a fish or an arthropod. Fortunately, there is still alive on the earth one member of this remarkable group—the Limulus, or King-Crab. On the vertebrate side the lowest non-degenerate vertebrate is the lamprey, or Petromyzon, which spends a large portion of its existence in a larval stage, known as the Ammocœtes stage of the lamprey, because it was formerly considered to be a separate species and received the name of Ammocœtes. The larval stages of any animal are most valuable for the study of ancestral problems, so that it is most fortunate for the solution of the ancestry of vertebrates that Limulus on the one side and Ammocœtes on the other are available for thorough investigation and comparison. There are no trilobites still alive, but in Branchipus and Apus we possess the nearest approach to the trilobite organization among living crustaceans.
So strongly do all these different lines of argument point to the origin of vertebrates from arthropods as to make it imperative to reconsider the position of that school of anatomists who derived vertebrates from annelids by reversing the back and front of the animal. Let us not turn the animal over, but re-consider the position, the infundibular tube of the vertebrate still representing the œsophagus of the invertebrate, the cerebral hemispheres and basal ganglia the supra-œsophageal ganglia, the crura cerebri the œsophageal commissures, and the infra-infundibular part of the brain the infra-œsophageal ganglia. It is immediately apparent that just as the invertebrate œsophagus leads into the large cephalic stomach, so the infundibular tube leads into the large cavity of the brain known as the third ventricle, which, together with the other ventricles, forms in the embryo a large anterior dilated part of the neural tube. In the arthropod this cephalic stomach leads into the straight narrow intestine; in the vertebrate the fourth ventricle leads into the straight narrow canal of the spinal cord. In the arthropod the intestine terminates in the anus; in the vertebrate embryo the canal of the spinal cord terminates in the anus by way of the neurenteric canal. Keep the animal unreversed, and immediately the whole mystery of the tubular nature of the central nervous system is revealed, for it is seen that the nervous matter, which corresponds bit by bit with that of the arthropod, has surrounded to a greater or less extent and amalgamated with the tube of the arthropod alimentary canal, and thus formed the so-called central nervous system of the vertebrate.
The manner in which the nervous material has invaded the walls of the tube is clearly shown both by the study of the comparative anatomy of the central nervous system in the vertebrate and also by its development in the embryo.
This theory implies that the vertebrate alimentary canal is a new formation necessitated by the urgency of the case, and, indeed, there was cause for urgency, for the general plan of the evolution of the invertebrate from the cœlenterate involved the piercing of the anterior portion of the central nervous system by the œsophagus, while, at the same time, upward progress meant brain-development; brain-development meant concentration of nervous matter at the anterior end of the animal, with the result that in the highest scorpion and spider-like animals the brain-mass has so grown round and compressed the food-tube that nothing but fluid pabulum can pass through into the stomach; the whole group have become blood-suckers. These kinds of animals—the sea-scorpions—were the dominant race when the vertebrates first appeared: here in the natural competition among members of the dominant race the difficulty must have become acute. Further upward evolution demanded a larger and larger brain with the ensuing consequence of a greater and greater difficulty of food-supply. Nature's mistake was rectified and further evolution secured, not by degeneration in the brain-region, for that means degradation not upward progress, but by the formation of a new food-channel, in consequence of which the brain was free to develop to its fullest extent. Thus the great and mighty kingdom of the Vertebrata was evolved with its culminating organism—man—whose massive brain with all its possibilities could never have been evolved if he had still been compelled to pass the whole of his food through the narrow œsophageal tube, still existent in him as the infundibular tube. This, then, is the working hypothesis upon which this book is written. If this view is right, that the Vertebrate was formed from the Palæostracan without any reversal of surfaces, but by the amalgamation of the central nervous system and alimentary canal, then it follows that we have various fixed points of comparison in the central nervous systems of the two groups of animals from which to search for further clues. It further follows that from such starting-point every organ of importance in the body of the arthropod ought to be visible in the corresponding position in the vertebrate, either as a functional or rudimentary organ. The subsequent chapters will deal with this detailed comparison of organs in the arthropod and vertebrate respectively.